The search for new therapeutic agents has been greatly aided in recent years by better understanding of the structure of enzymes and other biomolecules associated with target diseases. One important class of enzymes that has been the subject of extensive study is the protein kinases.
Protein kinases mediate intracellular signal transduction. They do this by effecting a phosphoryl transfer from a nucleoside triphosphate to a protein acceptor that is involved in a signaling pathway. There are a number of kinases and pathways through which extracellular and other stimuli cause a variety of cellular responses to occur inside the cell. Examples of such stimuli include environmental and chemical stress signals (e.g. osmotic shock, heat shock, ultraviolet radiation, bacterial endotoxin, H2O2), cytokines (e.g. interleukin-1 (IL-1) and tumor necrosis factor α (TNF-α)), and growth factors (e.g. granulocyte macrophage-colony-stimulating factor (GM-CSF), and fibroblast growth factor (FGF). An extracellular stimulus may effect one or more cellular responses related to cell growth, migration, differentiation, secretion of hormones, activation of transcription factors, muscle contraction, glucose metabolism, control of protein synthesis and regulation of cell cycle.
Many diseases are associated with abnormal cellular responses triggered by protein kinase-mediated events. These diseases include autoimmune diseases, inflammatory diseases, neurological and neurodegenerative diseases, cancer, cardiovascular diseases, allergies and asthma, Alzheimer's disease or hormone-related diseases. Accordingly, there has been a substantial effort in medicinal chemistry to find protein kinase inhibitors that are effective as therapeutic agents. A challenge has been to find protein kinase inhibitors that act in a selective manner. Since there are numerable protein kinases that are involved in a variety of cellular responses, non-selective inhibitors may lead to unwanted side effects.
AKT (also known as PKB or Rac-PK beta), a serine/threonine protein kinase, has been shown to be overexpressed in several types of cancer and is a mediator of normal cell functions [(Khwaja, A., Nature, 401, pp. 33–34, 1999); (Yuan, Z. Q., et al., Oncogene, 19, pp. 2324–2330, 2000); (Namikawa, K., et al., J. Neurosci., 20, pp. 2875–2886, 2000)]. AKT comprises an N-terminal pleckstrin homology (PH) domain, a kinase domain and a C-terminal “tail” region. Three isoforms of human AKT kinase (AKT-1, -2 and -3) have been reported so far [(Cheng, J. Q., Proc. Natl. Acad. Sci. USA, 89, pp. 9267–9271, 1992); (Brodbeck, D. et al., J. Biol. Chem. 274, pp. 9133–9136, 1999)]. The PH domain binds 3-phosphoinositides, which are synthesized by phosphatidyl inositol 3-kinase (PI3K) upon stimulation by growth factors such as platelet derived growth factor (PDGF), nerve growth factor (NGF) and insulin-like growth factor (IGF-1) [(Kulik et al., Mol. Cell. Biol., 17, pp. 1595–1606, 1997); (Hemmings, B. A., Science, 275, pp. 628–630, 1997)]. Lipid binding to the PH domain promotes translocation of AKT to the plasma membrane and facilitates phosphorylation by another PH-domain-containing protein kinases, PDK1 at Thr308, Thr309, and Thr305 for the AKT isoforms 1, 2 and 3, respectively. A second, as of yet unknown, kinase is required for the phosphorylation of Ser473, Ser474 or Ser472 in the C-terminal tails of AKT-1, -2 and -3 respectively, in order to yield a fully activated AKT enzyme.
Once localized to the membrane, AKT mediates several functions within the cell including the metabolic effects of insulin (Calera, M. R. et al., J. Biol. Chem., 273, pp. 7201–7204, 1998), induction of differentiation and/or proliferation, protein synthesisans stress responses (Alessi, D. R. et al., Curr. Opin. Genet. Dev., 8, pp. 55–62, 1998).
Manifestations of altered AKT regulation appear in both injury and disease, the most important role being in cancer. The first account of AKT was in association with human ovarian carcinomas where expression of AKT was found to be amplified in 15% of cases (Cheng, J. Q. et al., Prod. Natl. Acad. Sci. U.S.A., 89, pp. 9267–9271, 1992). It has also been found to be overexpressed in 12% of pancreatic cancers (Cheng, J. Q. et al., Proc. Natl. Acad. Sci. U.S.A., 93, pp. 3636–3641, 1996). It was demonstrated that AKT-2 was over-expressed in 12% of ovarian carcinomas and that amplification of AKT was especially frequent in 50% of undifferentiated tumours, suggesting that AKT may also be associated with tumour aggressiveness (Bellacosa, et al., Int. J. Cancer, 64, pp. 280–285, 1995).
PKA (also known as cAMP-dependent protein kinase) has been shown to regulate many vital functions including energy metabolism, gene transcription, proliferation, differentiation, reproductive function, secretion, neuronal activity, memory, contractility and motility (Beebe, S. J., Semin. Cancer Biol., 5, pp. 285–294, 1994). PKA is a tetrameric holoenzyme, which contains two catalytic subunits bound to a homo-dimeric regulatory subunit (which acts to inhibit the catalytic sub-units). On binding of cAMP (enzyme activation), the catalytic subunits dissociate from the regulatory subunits to yield the active serine/threonine kinase (McKnight, G. S. et al., Recent Prog. Horm. Res., 44, pp. 307, 1988). Three isoforms of the catalytic subunit (C-α, C-β and C-γ have been reported to date (Beebe, S. J. et al., J. Biol. Chem., 267, pp. 25505–25512, 1992) with the C-α subunit being the most extensively studied, primarily because of its elevated expression in primary and metastatic melanomas (Becker, D. et al., Oncogene, 5, pp. 1133, 1990). To date, strategies to modulate the activity of the C-α subunit involve the use of antibodies, molecules that block PKA activity by targeting regulatory dimers and antisense oligonucleotides expression.
Rho-associated coiled-coil forming kinase (ROCK) (Ishizaki, T. et al., EMBO J., 15, pp. 1885–1893, 1996) is a 160 kDa serine/threonine kinase that activates the small G-protein RhoA. ROCK has been implicated in numerous diseases including hypertension [(Chitaley, et al., Curr. Hypertens. Rep. 2001 Apr., 3(2), pp.139–144); (Uehata, M. et al., Nature, 389, pp. 990–994, 1997)], erectile dysfunction (Chitaley, K. et al., Nature Medicine, 7, pp. 119–122, 2001), angiogenesis (Uchida, S. et al., Biochem. Biophys. Res. Commun., 269 (2), pp. 633–40, 2000), neuroregeneration (Bito, H. et al., Neuron, 26, pp. 431–441, 2000), metastasis [(Takamura, M. et al., Hepatology, 33, pp. 577–581, 2001); (Genda, T. et al., Hepatology, 30, pp. 1027–1036, 1999)], glaucoma (Rao, et al., Invest. Ophthalmol. Vis. Sci., 42, pp. 1029–37, 2001), inflammation (Ishizuka, T. et al., J. Immunol., 167, pp. 2298–2304, 2001), arteriosclerosis (Smimokawa, et al., Arterioscler. Thromb. Vasc. Biol., 11, pp. 2351–2358, 2000), immunosuppresion (Lou, Z. et al., J. Immunol., 167, pp. 5749–5757, 2001), restenosis (Seaholtz, et al., Circ. Res., 84, pp. 1186–1193, 1999), asthma (Yoshii, et al., Am. J. Respir. Cell Mol. Biol., 20, pp. 1190–1200, 1999), cardiac hypertrophy (Kuwahara, K. et al., FEBS Lett., 452, pp. 314–318, 1999).
The ribosomal protein kinases p70S6K-1 and -2 are members of the AGC sub-family of protein kinases that consists of, amongst others, PKB and MSK. The p70S6 kinases catalyze the phosphorylation and subsequent activation of the ribosomal protein S6, which has been implicated in the translational up-regulation of mRNAs coding for the components of protein synthetic apparatus.
These mRNAs contain an oligopyrimidine tract at their 5′ transcriptional start site, termed a 5TOP, which has been shown to be essential for their regulation at the translational level (Volarevic, S. et al., Prog. Nucleic Acid Res. Mol. Biol. 65, pp 101–186, 2001). p70 S6K dependent S6 phosphorylation is stimulated in response to a variety of hormones and growth factors primarily via the P13K pathway (Coffer, P. J. et al., Biochem. Biophys. Res. Commun, 198, 7 pp 780–786, 1994), which maybe under the regulation of mTOR, since rapamycin acts to inhibit p70S6K activity and blocks protein synthesis, specifically as a result of a down-regulation of translation of these mRNA's encoding ribosomal proteins (Kuo, C. J. et al., Nature, 358, pp 70–73, 1992).
In vitro PDK1 catalyses the phosphorylation of Thr252 in the activation loop of the p70 catalytic domain, which is indispensable for p70 activity (Alessi, D. R., Curr. Biol., 8, pp 69–81, 1998). The use of rapamycin and gene deletion studies of dp70S6K from Drosophila and p70S6K1 from mouse have established the central role p70 plays in both cell growth and proliferation signaling.
The 3-phosphoinositide-dependent protein kinase-1 (PDK1) plays a key role in regulating the activity of a number of kinases belonging to the AGC subfamily of protein kinases (Alessi, D. et al., Biochem. Soc. Trans, 29, pp. 1, 2001). These include isoforms of protein kinase B (PKB, also known as AKT), p70 ribosomal S6 kinase (S6K) (Avruch, J. et al., prog. Mol. Subcell. Biol., 2001, 26, pp. 115, 2001), and p90 ribosomal S6 kinase (Frödin, M. et al., EMBO J., 19, pp. 2924–2934, 2000). PDK1 mediated signaling is activated in response to insulin and growth factors and as a consequence of attachment of the cell to the extracellular matrix (integrin signaling). Once activated these enzymes mediate many diverse cellular events by phosphorylating key regulatory proteins that play important roles controlling processes such as cell survival, growth, proliferation and glucose regulation [(Lawlor, M. A. et al., J. Cell Sci., 114, pp. 2903–2910, 2001), (Lawlor, M. A. et al., EMBO J., 21, pp. 3728–3738, 2002)]. PDK1 is a 556 amino acid protein, with an N-terminal catalytic domain and a C-terminal pleckstrin homology (PH) domain, which activates its substrates by phosphorylating these kinases at their activation loop (Belham, C. et al., Curr. Biol., 9, pp. R93–R96, 1999). Many human cancers including prostate and NSCL have elevated PDK1 signaling pathway function resulting from a number of distinct genetic events such as PTEN mutations or over-expression of certain key regulatory proteins [(Graff, J. R., Expert Opin. Ther. Targets, 6, pp. 103–113, 2002), (Brognard, J., et al., Cancer Res., 61, pp. 3986–3997, 2001)]. Inhibition of PDK1 as a potential mechanism to treat cancer was demonstrated by transfection of a PTEN negative human cancer cell line (U87MG) with antisense oligonucleotides directed against PDK1. The resulting decrease in PDK1 protein levels led to a reduction in cellular proliferation and survival (Flynn, P., et al., Curr. Biol., 10, pp. 1439–1442, 2000). Consequently the design of ATP binding site inhibitors of PDK1 offers, amongst other treatments, an attractive target for cancer chemotherapy.
The diverse range of cancer cell genotypes has been attributed to the manifestation of the following six essential alterations in cell physiology: self-sufficiency in growth signaling, evasion of apoptosis, insensitivity to growth-inhibitory signaling, limitless replicative potential, sustained angiogenesis, and tissue invasion leading to metastasis (Hanahan, D. et al., Cell, 100, pp. 57–70, 2000). PDK1 is a critical mediator of the P13K signalling pathway, which regulates a multitude of cellular function including growth, proliferation and survival. Consequently inhibition of this pathway could affect four or more of the six defining requirements for cancer progression, as such it is anticipated that a PDK1 inhibitor will have an effect on the growth of a very wide range of human cancers.
Specifically, increased levels of PI3K pathway activity has been directly associated with the development of a number of human caners, progression to an aggressive refractory state (acquired resistance to chemotherapies) and poor prognosis. This increased activity has been attributed to a series of key events including decreased activity of negative pathway regulators such as the phosphatase PTEN, activating mutations of positive pathway regulators such as Ras, and overexpression of components of the pathway itself such as PKB, examples include: brain (gliomas), breast, colon, head and neck, kidney, lung, liver, melanoma, ovarian, pancreatic, prostate, sarcoma, thyroid [(Teng, D. H. et al., Cancer Res., 57, pp. 5221–5225, 1997), (Brognard, J. et al., Cancer Res., 61, pp. 3986–3997, 2001), (Cheng, J. Q. et al., Proc. Natl. Acad. Sci., 93, pp. 3636–3641, 1996), Int. J. Cancer, 64, pp. 280, 1995), (Graff, J. R., Expert Opin. Ther. Targets, 6, pp. 103–113, 2002), Am. J. Pathol., 159, pp. 431, 2001)].
Additionally, decreased pathway function through gene knockout, gene knockdown, dominant negative studies and small molecule inhibitors of the pathway have been demonstrated to reverse many of the cancer phenotypes in vitro (some studies have also demonstrated a similar effect in vivo) such as block proliferation, reduce viability and sensitize cancer cells to known chemotherapies in a series of cell lines, representing the following cancers: pancreatic [(Cheng, J. Q. et al., Proc. Natl. Acad. Sci., 93, pp. 3636–3641, 1996), Neoplasia, 3, pp. 278, 2001)], lung [(Brognard, J. et al., Cancer Res., 61, pp. 3986–3997, 2001), Neoplasia, 3, pp. 278, 2001)], ovarian [(Hayakawa, J. et al., Cancer Res., 60, pp. 5988–5994, 2000), Neoplasia, 3, pp. 278, 2001)], breast (Mol. Cancer Ther., 1, pp. 707, 2002), colon [(Neoplasia, 3, pp. 278, 2001), (Arico, S. et al., J. Biol. Chem., 277, pp. 27613–27621, 2002)], cervical (Neoplasia, 3, pp. 278, 2001), prostate [(Endocrinology, 142, pp. 4795, 2001), (Thakkar, H. et al. J. Biol. Chem., 276, pp. 38361–38369, 2001), (Chen, X. et al., Oncogene, 20, pp. 6073–6083, 2001)] and brain (glioblastomas) [(Flynn, P. et al., Curr. Biol., 10, pp. 1439–1442, 2000)].
Accordingly, there is a great need to develop inhibitors of AKT, PKA, PDK1, p70S6K, and ROCK protein kinases that are useful in treating various diseases or conditions associated with AKT, PKA, PDK1, p70S6K, and ROCK activation, particularly given the inadequate treatments currently available for the majority of these disorders.